I am investigating the control of cell division, my specific goals being the identification of the genes involved, the elucidation of their molecular products, and ultimately the determination of the physiological roles that these play. Yeast (Saccharomyces cerevisiae), a simple eukaryotic organism amenable to both genetic and biochemical investigations, will be employed for the proposed research. In yeast, division is normally controlled in response to nutrient limitation and in preparation for conjugation. Both forms of control are integrated in the G1 interval at a step of the cell cycle known as start. Undoubtedly many gene products are directly involved and eludication of their individual roles would be a major step in claritying how control of cell division in eukaryotes is organized. In order to identify the relevant genes, mutants which cannot complete the start step were sought. To date, I have isolated 40 independent mutations and assigned them to four unlinked complementation groups. These genes have now been isolated physically by recombinant DNA techniques and may be used as reagents in the characterization of the corresponding mRNAs and polypeptides that they encode. Antisera reactive against these polypeptides will be prepared using novel chimeric genes which can be expressed at high level in E. coli. The intracellular location, the metabolism and the physiological roles of the start genes products will be investigated.